1. Field of the Invention
The present invention relates to an optical information reproducing apparatus for reproducing information recorded as a mark string on a recording medium by irradiating a light beam thereon and, more particularly, to a signal processing circuit for processing a reproducing signal in an optical information reproducing apparatus.
2. Description of the Related Art
An optical information reproducing apparatus (e.g., an optical disk apparatus using a disk-like recording medium called an optical disk) is designed to record information as mark strings on an optical disk along helical (or concentrical) tracks, and optically reproduce the recorded information by irradiating a light beam such as a laser beam thereon and detecting reflection/diffraction light.
As apparatuses allowing users to record information signals on optical disks with laser beams, document file systems and the like have already been commercialized, and products used as peripheral devices of computers, which demand higher reliability, have also been put into practice. In addition, apparatuses and the like which can erase recorded information and write new information have been put into practice. Optical card memory devices and optical tape memory devices, which are obtained by applying the same technique to card- and tape-like recording media, are being developed. Furthermore, compact disks, CD-ROMs, laser disks, and the like have been put into practice as read-only optical disks.
The above optical information reproducing apparatus reproduces information by tracking a recorded mark string having a size of about 1 .mu.m, and a track pitch of about 1.6 .mu.m with, e.g., a laser beam having an optical spot diameter focused to about 1.2 .mu.m. As schemes of recording marks, various schemes have been proposed and put into practice. For example, in a read-only optical disk, when a substrate is molded by a stamper, recesses/projections called pits are locally formed. In an optical disk of the type which allows the user to record information, a recording film is subjected to changes in optical characteristics due to local destruction, deformation, or phase change.
Similar to other storage devices, as the application field of these optical information reproducing apparatus expands, an increase in capacity and a decrease in size have been required. For this reason, attempts have been made to attain higher recording densities.
As methods of increasing the recording density, a method of increasing the line recording density and a method of increasing the track density are available. These methods, however, have their own limits. More specifically, in order to increase the line recording density, the period of a fundamental clock (to be referred to as a channel clock hereinafter) in the recording mode may be shortened. This, however, increases waveform deformation due to intersymbol interference of a reproducing signal. For this reason, it is difficult to keep the error rate within an allowable range. In order to increase the track density, the track pitch may be decreased. This, however, increases the influence of crosstalk between adjacent tracks on reproducing signals. For this reason, it is also difficult to set the error rate in the allowable range.
As described above, in the conventional optical information reproducing apparatus, if the period of a fundamental clock in the recording mode is shortened to increase the line recording density, the waveform deformation due to intersymbol interference of a reproducing signal increases. If the track pitch is decreased to increase the track density, the influence of crosstalk between adjacent tracks on a reproducing signal increases. In either case, it is difficult to keep the error rate in an allowable range.
The above problem is noticeable especially when an optical disk has a tilt (e.g., a relative tilt with respect to a light beam). Although no consideration has been given to the tilt of an optical disk, the influence of the tilt of the optical disk is large, like intersymbol interference and crosstalk. For this reason, the schemes of attaining higher recording densities by increasing the line recording density and the track density have their own limits.